Conventionally, in communication devices such as mobile phones, filter devices are used to separate signals having different bands such as a transmission signal and a reception signal. Electronic devices including bulk acoustic wave (BAW) resonators, such as a film bulk acoustic resonators (FBAR) and solidly mounted resonators (SMRs), have been used in filter devices. Such electronic devices may include a device substrate on which an electronic circuit is disposed and a cap substrate. Such electronic devices may be manufactured as follows: portions to be bonded between the device substrate and the cap substrate are formed with the same types of metals such as gold or copper; the metal portions are covalently bonded with each other at high temperature and high pressure; and then the device substrate and the cap substrate are bonded together.
Background material describing FBAR filters and Surface Acoustic Wave (SAW) filters includes “Development of FBAR Filters: In Comparison with SAW Filters,” Transactions of Institute of Electronics, Information and Communication Engineers, Electronic Device, 103(728), 9-14, 2004 Mar. 9.